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The chemistry and radiation on the day side of hot giant exoplanets can be better explained by the physics of stars than that of planets. This is the conclusion of research on the exoplanet WASP-18b, led by astrophysicists at the University of Amsterdam. This new finding calls for a review of data from dozens of other exoplanets in search of new information. This research was recently published in The Astrophysical Journal Letters.

2600 degrees Celcius

The physicists analyzed the light from WASP-18b, one of the hottest and most massive giant exoplanets we know. The planet can be found at a distance of 400 light years from Earth, is ten times as heavy as Jupiter, and is very close to its star. It revolves around the star every 23 hours and,  since it's so close to its host star, it is tidally locked, like the Moon is tidally locked to the Earth and shows always the same hemisphere to us. This is why the planet is said to have a day side and a night side.

The researchers observed the planet during five stellar occultations, using the Hubble Space Telescope. With the so-called emission spectrum, they could determine the temperature and the composition of the planet's atmosphere. The day side appears to be around 2600 degrees Celsius and the planet's atmosphere looks more like that of a star than that of a planet. 

Giant exoplanet WASP-18b is 410 light years away from earth and revolves very closely around its sun.

Metallic hydrogen

The observed data do not in the slightest agree with existing models for cool exoplanets. Only when the researchers extended existing models with theories used for stars, the models began to match the data. First author Jacob Arcangeli (UvA) explains: 'We found that we had to take into account the fact that water molecules are broken apart under extreme temperatures. Our models also consider that hydrogen can be temporarily ionized. This allows for the hydrogen to acquire metallic properties which in turn makes the atmosphere more opaque. These new findings give us a new understanding of previous observations, which will very likely yield a deeper insight into the origins of these massive, close in planets.'

Group leader Jean-Michel Désert (UvA): 'A thorough understanding of planetary atmospheres is essential to understand how and where planets are formed.'

Publication details:

H− Opacity and Water Dissociation in the Dayside Atmosphere of the Very Hot Gas Giant WASP-18b. Door: Jacob Arcangeli, Jean-Michel Désert, Michael R. Line, Jacob L. Bean, Vivien Parmentier, Kevin B. Stevenson, Laura Kreidberg, Jonathan J. Fortney, Megan Mansfield & Adam P. Showman. In: The Astrophysical Journal Letters, Volume 855, Number 2, March 15, 2018.

Link to the article